feat(logging): add Loggers registry (#737)

Part 6 of the logging stack (builds on #726). Lets applications pick a logging backend from configuration and install it as the process default — the same pattern as `MetricsReporters`.

**How it works**

Choose a backend with the `logger-impl` property; built-ins are `noop`, `cerr`, and `spdlog` (when compiled in).

```cpp
// Build a logger from properties and install it as the process default.
auto logger = iceberg::Loggers::Load({{"logger-impl", "spdlog"}, {"level", "info"}});
if (logger) {
  iceberg::SetDefaultLogger(
      std::shared_ptr<iceberg::Logger>(std::move(*logger)));
}

// ...then anywhere in the code:
ICEBERG_LOG_INFO("scan planned: {} files", n);
```

**API**
- `Loggers::Load(properties)` — build a logger, choosing the type from `logger-impl`.
- `Loggers::Register(type, factory)` — register a custom backend under a new name.

With no `logger-impl` set, the default is `spdlog` when compiled in, otherwise `cerr` — so logging works out of the box.

**Tests** — `loggers_test` drives the full path an app uses: configure a backend + level via properties, install it, log through the macros, and check the real output. Covers configured level handling and a macro reaching a real spdlog sink, with spdlog ON and OFF.

This pull request and its description were written by Isaac.

Author: kamcheungting-db

src/iceberg/CMakeLists.txt

@@ -57,6 +57,7 @@ set(ICEBERG_SOURCES
     location_provider.cc
     logging/cerr_logger.cc
     logging/logger.cc
+    logging/loggers.cc
     logging/spdlog_logger.cc
     manifest/manifest_adapter.cc
     manifest/manifest_entry.cc

src/iceberg/logging/logger.cc

@@ -83,7 +83,7 @@ struct ThreadCache {
 std::shared_ptr<Logger> Logger::Noop() {
   // Intentionally leaked: reachable via the function-local static (LSan-clean)
   // and never destroyed, so logging during static teardown stays safe.
-  static auto* instance = new std::shared_ptr<Logger>(std::make_shared<NoopLogger>());
+  static auto* instance = new std::shared_ptr<Logger>(internal::MakeNoopLogger());
   return *instance;
 }
 
@@ -141,6 +141,8 @@ FatalHandler GetFatalHandler() {
 
 namespace internal {
 
+std::unique_ptr<Logger> MakeNoopLogger() { return std::make_unique<NoopLogger>(); }
+
 namespace {
 
 /// \brief The one place the per-thread cache's lifetime is managed; shared by

src/iceberg/logging/logger.h

@@ -296,6 +296,10 @@ class ICEBERG_EXPORT ScopedLogger {
 
 namespace internal {
 
+/// \brief Construct a fresh no-op logger. Shared by Logger::Noop() (which caches a
+/// single instance) and the "noop" registry factory (which needs an owned one).
+ICEBERG_EXPORT std::unique_ptr<Logger> MakeNoopLogger();
+
 /// \brief Hot-path accessor for the default logger.
 ///
 /// Returns a reference to a thread-local cached shared_ptr that is refreshed

src/iceberg/logging/loggers.cc

@@ -0,0 +1,131 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+#include "iceberg/logging/loggers.h"
+
+#include <exception>
+#include <memory>
+#include <mutex>
+#include <shared_mutex>
+#include <string>
+#include <unordered_map>
+#include <utility>
+
+// Build-generated, .cc-only. Defines ICEBERG_HAS_SPDLOG; tested with #ifdef.
+#include "iceberg/logging/cerr_logger.h"
+#include "iceberg/logging/config.h"
+#include "iceberg/util/macros.h"
+#ifdef ICEBERG_HAS_SPDLOG
+#  include "iceberg/logging/spdlog_logger_internal.h"
+#endif
+
+namespace iceberg {
+
+namespace {
+
+/// \brief Extract the logger type, defaulting to the compiled-in backend.
+std::string InferLoggerType(
+    const std::unordered_map<std::string, std::string>& properties) {
+  auto it = properties.find(std::string(kLoggerImpl));
+  if (it != properties.end() && !it->second.empty()) {
+    return it->second;
+  }
+#ifdef ICEBERG_HAS_SPDLOG
+  return std::string(kLoggerTypeSpdlog);
+#else
+  return std::string(kLoggerTypeCerr);
+#endif
+}
+
+struct LoggerRegistryState {
+  std::shared_mutex mtx;
+  std::unordered_map<std::string, LoggerFactory> map;
+};
+
+LoggerRegistryState& GetRegistry() {
+  static auto* state = new LoggerRegistryState{
+      .map = {
+          {std::string(kLoggerTypeNoop),
+           [](const std::unordered_map<std::string, std::string>&)
+               -> Result<std::unique_ptr<Logger>> { return internal::MakeNoopLogger(); }},
+          {std::string(kLoggerTypeCerr),
+           [](const std::unordered_map<std::string, std::string>&)
+               -> Result<std::unique_ptr<Logger>> {
+             return std::make_unique<CerrLogger>();
+           }},
+#ifdef ICEBERG_HAS_SPDLOG
+          {std::string(kLoggerTypeSpdlog),
+           [](const std::unordered_map<std::string, std::string>&)
+               -> Result<std::unique_ptr<Logger>> {
+             return std::make_unique<internal::SpdLogger>();
+           }},
+#endif
+      }};
+  return *state;
+}
+
+}  // namespace
+
+Status Loggers::Register(std::string_view logger_type, LoggerFactory factory) {
+  if (!factory) {
+    return InvalidArgument("Logger factory for '{}' must not be empty", logger_type);
+  }
+  auto& registry = GetRegistry();
+  std::unique_lock lock(registry.mtx);
+  registry.map[std::string(logger_type)] = std::move(factory);
+  return {};
+}
+
+Result<std::unique_ptr<Logger>> Loggers::Load(
+    const std::unordered_map<std::string, std::string>& properties) {
+  std::string logger_type = InferLoggerType(properties);
+
+  LoggerFactory factory;
+  {
+    auto& registry = GetRegistry();
+    std::shared_lock lock(registry.mtx);
+    auto it = registry.map.find(logger_type);
+    if (it == registry.map.end()) {
+      return InvalidArgument(
+          "Unknown logger type '{}'. Register a factory with Loggers::Register() "
+          "before using this type.",
+          logger_type);
+    }
+    factory = it->second;
+  }
+
+  try {
+    // Run the (user-supplied) factory outside the registry lock so it cannot
+    // deadlock or re-enter the registry; the try/catch turns a throwing factory
+    // into an error instead of propagating.
+    ICEBERG_ASSIGN_OR_RAISE(auto logger, factory(properties));
+    if (!logger) {
+      return InvalidArgument("Logger factory for '{}' returned null", logger_type);
+    }
+    ICEBERG_RETURN_UNEXPECTED(logger->Initialize(properties));
+    return logger;
+  } catch (const std::exception& ex) {
+    return InvalidArgument("Logger factory for '{}' failed: {}", logger_type, ex.what());
+  } catch (...) {
+    return InvalidArgument("Logger factory for '{}' failed with unknown exception",
+                           logger_type);
+  }
+}
+
+}  // namespace iceberg

src/iceberg/logging/loggers.h

@@ -0,0 +1,64 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+#pragma once
+
+/// \file iceberg/logging/loggers.h
+/// \brief Property-driven registry/factory for Logger backends.
+
+#include <functional>
+#include <memory>
+#include <string>
+#include <string_view>
+#include <unordered_map>
+
+#include "iceberg/iceberg_export.h"
+#include "iceberg/logging/logger.h"
+#include "iceberg/result.h"
+
+namespace iceberg {
+
+/// \brief Property key selecting the logger implementation.
+constexpr std::string_view kLoggerImpl = "logger-impl";
+/// \brief Built-in logger type identifiers.
+constexpr std::string_view kLoggerTypeNoop = "noop";
+constexpr std::string_view kLoggerTypeCerr = "cerr";
+constexpr std::string_view kLoggerTypeSpdlog = "spdlog";
+
+/// \brief Factory constructing a Logger from catalog-style properties.
+using LoggerFactory = std::function<Result<std::unique_ptr<Logger>>(
+    const std::unordered_map<std::string, std::string>& properties)>;
+
+/// \brief Registry of logger factories, mirroring MetricsReporters.
+///
+/// Built-in factories: "noop", "cerr", and (only when built with ICEBERG_SPDLOG)
+/// "spdlog". When the "logger-impl" property is absent, the default is "spdlog"
+/// if compiled in, otherwise "cerr" -- an intentional divergence from the metrics
+/// registry's noop default (we want logs by default).
+class ICEBERG_EXPORT Loggers {
+ public:
+  /// \brief Construct and initialize a logger from properties.
+  static Result<std::unique_ptr<Logger>> Load(
+      const std::unordered_map<std::string, std::string>& properties);
+
+  /// \brief Register a factory for \p logger_type (overwrites any existing).
+  static Status Register(std::string_view logger_type, LoggerFactory factory);
+};
+
+}  // namespace iceberg

src/iceberg/logging/meson.build

@@ -29,6 +29,7 @@ install_headers(
         'log_level.h',
         'log_macros.h',
         'logger.h',
+        'loggers.h',
         'short_log_macros.h',
     ],
     subdir: 'iceberg/logging',

src/iceberg/meson.build

@@ -109,6 +109,7 @@ iceberg_sources = files(
     'location_provider.cc',
     'logging/cerr_logger.cc',
     'logging/logger.cc',
+    'logging/loggers.cc',
     'logging/spdlog_logger.cc',
     'manifest/manifest_adapter.cc',
     'manifest/manifest_entry.cc',

src/iceberg/test/CMakeLists.txt

@@ -96,6 +96,7 @@ add_iceberg_test(logging_test
                  cerr_logger_test.cc
                  log_level_test.cc
                  logger_test.cc
+                 loggers_test.cc
                  macros_active_level_test.cc
                  macros_test.cc
                  spdlog_logger_test.cc)